Undersea communication options and their accompanying constraints are significantly different than options and constraints encountered above the surface. In particular, radio frequency (RF) communications are severely limited undersea as electro-magnetic waves are rapidly absorbed in a conductive medium like seawater. A widely used option for undersea communications is acoustics. Acoustics may be considered similar to RF communications, but at acoustic frequencies. Acoustic pressure waves propagate very well in water with reasonably low loss. However, because acoustic frequencies are much lower than RF frequencies, the corresponding data rate is also much lower. Another disadvantage is that acoustic communication does not exhibit low probability of intercept (LPI), as the acoustic signals can spread out and travel long distances. Thus, undersea RF or acoustics fall far short in meeting undersea operational requirements for connectivity due to physics, the lack of LPI, insufficient bandwidth and/or the ability to be jammed or otherwise denied.
Undersea free-space optical communication, typically laser communication, has been considered to overcome some of these limitations. Propagation in sea water is best for blue light in deep water and green light in shallower water, with ranges typically on the order of a few hundred meters in deep clear dark water, and much less near the surface where turbidity of the water can greatly limit the range. Attenuation occurs due to losses from absorption and scattering, which is enhanced by the effects of turbidity. Scattering also increases the divergence of a transmitted laser beam. Scattering decreases the received signal level and makes a wider field-of-view necessary at the receiver. Also, scattering causes multipath due to the path delay differences. Multipath limits the data rate of single carrier systems as the pulses temporally disperse and data symbols interfere with each other, creating inter-symbol interference (ISI). Modern lasers can provide Gigabit per second (Gbps) data rates at a range of about 100-200 meters (m). This data rate is ideal for uploading or downloading massive amounts of data quickly over short distances. At longer ranges, much lower data rates are achievable (e.g., only 100's of bits-per-second (bps) data rates) at 1 km using a 1 meter diameter receiver and 1 Watt transmitter. Today's systems typically require higher data rates. Laser communications are also highly directional. This requires techniques for accurate pointing along with acquisition of the pointing angle. This directionality also provides for some LPI, as does the range limitation. Optical systems are also less prone to jamming and eavesdropping.
A need exists to dramatically increase the data rate and range of undersea laser communications, increasing undersea connectivity and robustness against turbidity, while mitigating multipath ISI and providing for better LPI and protection against eavesdropping.